Engine generators including diesel engines or the like are conventionally used for a power source driving an electric motor such as a motor or a pump in cranes or the like. The engine generator supplies generated electric power to the electric motor through a converter and an inverter in the crane or the like. A capacitor such as a battery is also provided on a circuit supplying electric power to the electric motor in parallel with the inverter (for example, Patent Documents 1 and 2).
In techniques of Patent Documents 1 and 2, energy efficiency can be increased because surplus power can be stored in the capacitor when an electric motor requires a small amount of electric power. Then, both of the engine generator and the capacitor can supply electric power to the electric motor when the electric motor requires a large amount of electric power. Because of this, there is an advantage that the engine generator can be downsized.
However, a load applied to the engine generator varies in proportion to the electric power required by the electric motor in the techniques of Patent Documents 1 and 2. Because of this, the load applied to the engine generator significantly increases if the electric power required by the electric motor significantly increases, so that there is a possibility that a momentary voltage of the engine generator drops. The engine generator may stop in extreme cases.
As a technique solving the above problem, a technique is also developed in which all of the electric power generated by the engine generator is supplied to a battery and the electric power is supplied to a motor or the like only by the battery (Patent Document 3).
In the technique of Patent Document 3, since the engine generator is only used for charging the battery, the engine generator can be operated in an optimal condition regardless of electric power required by the electric motor. Because of this, there is an advantage of being able to improve fuel consumption.
In the case of the technique of Patent Document 3, a load of the engine generator does not vary if the electric power required by the motor varies because the electric power is only supplied from the battery. However, there is a problem that a power supply device increases in size because a large battery is required for certainly supplying the electric power required by the motor.
Patent Document 1
Japanese Patent Laid-Open No. 11-217193
Patent Document 2
Japanese Patent Laid-Open No. 11-285165
Patent Document 3
Japanese Patent Laid-Open No. 2003-102106